logging / ring buffer for ALERT messages from A-bis OML from the BTS side

OML ALERT messages are currently not really used much. They should be used more from the BTS side, and the BSC component in the NITB should not only log them somewhere but possibly even keep an in-memory rng buffer for each BTS/TRX so one can inspect the most recent errors for those objects via the VTY.

This is actually slightly similar to the "show last unsuccessful BTS connection attempts". In both cases we want to keep a ring-buffer of most recent events. However, the difference is that for ALERT, we want one FIFO ring buffer per BTS, while for the BTS connection attempts, we want one global ring buffer, and entries should be overwritten in case the same BTS attempts again...